LITERATURE REVIEW
2.4 BENTONITE .1 Introduction
Bentonite is widely used as a backfill material during the construction of slurry trench walls, as a soil admixture for the construction of seepage barriers, as a grout material, as a sealant for piezometer installations and for various other civil engineering construction techniques. Bentonite is an absorbent aluminium phyllosilicate, essentially impure clay, formed as a deposit of volcanic ashes at shallow wet sites in various location of the world (Grim & Guven, 1978). These deposits are variable, depending on the nature of the volcanic ashes and the salinity of the water into which they were deposited. Since the bentonite is a natural material, its mineral composition, chemical state, and grain size distribution varies considerably from one source to another. Different parameters such as mineralogical composition (i.e. amount and type of montmorillonite), type of exchangeable cations, surface area and the surface charge density affect the behaviour of bentonite considerably.
Bentonite is primarily composed of the smectite group of minerals, most common among which is montmorillonite (Al1.7Mg0.3)[Si4O10(OH)2]-0.3(M)+0.3, where M represents the exchangeable cation (Mitchell and Soga, 2005). The behaviour of bentonite primarily is governed by montmorillonite which has characteristics like a large specific surface area (as high as 800 m2/g), high charge deficiency (0.5-1.2 per unit cell), high cation exchange capacity (80-150 cmolc/kg), and ability for interlayer swelling. These factors contribute to the high swelling, low hydraulic conductivity and contaminants adsorption ability of the bentonite.
2.4.2 Structure of Montmorillonite
Clays are the particles with an effective diameter smaller than 2m and phyllosilicates as its main mineralogical components. These phyllosilicates are made of silica (SiO2) tetrahedral sheets and Aluminium (Al3+) or magnesium (Mg2+) oxides octahedral sheets.
Montmorillonite has a prototype structure similar to that of pyrophyllite consisting of an octahedral sheet sandwiched between two tetrahedral sheets (2:1 mineral) and diagrammatically in three dimensions (Fig. 2.3). The silica and gibbsite sheets are combined in such a way that the tips of the tetrahedron of each silica sheet and one of hydroxyl layers of octahedral sheet form a common layer and all the tips of the tetrahedral point toward the center of the unit cell. The oxygen forming the tips of the tetrahedral is shared with the octahedral sheet as well. The anions in the octahedral sheet that fall directly above and below the hexagonal holes formed by the bases of the silica tetrahedral are hydroxyls. Bonding between successive layers is by van der Waal’s forces and by cations that balance charge deficiencies in the structure. These bonds are weak and water or other polar liquids can easily enter between the layers, causing them to expand significantly. It has a lateral dimension of 1000 to 5000 A0 and thickness 10 to 50 A0.
The layers formed in this way are continuous in ‘a’ and ‘b’ directions and stacked one above the other in the ‘c’ direction. Bonding between successive layers is by van der Waal’s forces and by cations that balance the charge deficiencies in the structure. These bonds are weak and easily separated by cleavage or adsorption of water or other polar liquids. The basal spacing in the c direction, d(001), is variable, ranging from about 0.96 nm (1 nm = 10-6 mm) to complete separation.
Figure 2.3 Structure of montmorillonite (Mitchell and Soga, 2005)
The montmorillonite is the primary mineral of bentonite. In the dry state a particle of montmorillonite resembles a closed book composed of many thin crystalline sheets held
9.6 A0 to
nH2O+cations in interlayer regions
together by weak van der Waal’s forces and by cations. Each sheet has charge deficiencies within its crystal structure, and is neutralized by the presence of cations held loosely to the surface of the sheets. When the dry bentonite and water are mixed, water is drawn into the montmorillonite particles to hydrate the surface of the elemental sheets and the cations. For the combination of sodium montmorillonite and freshwater, the fluid that enters the particles forms thick, viscous diffuse ionic layers around the layer, causing the montmorillonite particles to swell, possibly to the extent of complete separation of the sheets. The fabric of freshwater, low salt, sodium bentonite resembles a pile of crumbled paper. For the combination of dry sodium bentonite and a saline solution, less fluid is required to neutralize the negatively charged sheets, and if the ion concentration is large or the valence of the cations are large, the separation distance between sheets will remain small and the montmorillonite particles will remain in the form of closed books. The fabric of bentonite in this case will consist of swollen but intact montmorillonite particles surrounded by thin, viscous diffuse ionic layers, in an arrangement resembling a pile of fallen books. A third case is that of calcium bentonite, an example of bentonite in which dominant exchangeable cations is polyvalent. The calcium cation is very effective in holding together the montmorillonite sheets, and therefore calcium bentonite has small potential to swell, even when mixed with freshwater. Calcium bentonite behaves similarly to sodium bentonite in a high salt state, and its permeability properties are about same.
2.4.3 Swelling behaviour of Bentonite
The swelling of bentonite takes place in two stages, inner-crystalline swelling and osmotic swelling (Norrish and Quirk, 1954).
2.4.3.1 Inner-crystalline swelling
In inner-crystalline swelling, water molecules enter the interlayer region of the montmorillonite to hydrate the exchangeable cations located there. The cations upon contact with water order themselves on a plane halfway between the clay layers which lead to a widening of the spacing between the layers. The volume of montmorillonite can double in the process of inner-crystalline swelling. Polarity of the water molecule is an important factor in the inner-crystalline swelling of clay. When cations hydrate, the water molecules orient their negative dipoles towards the cation and thus weaken the electrostatic interaction between the negatively charged layers and the interlayer cations.
Inner-crystalline swelling, which has also been referred to as Type I swelling, is a process whereby expandable 2:1 phyllosilicates sequentially intercalate one, two, three or four discrete layers of H2O molecules between the mineral interlayers (Norrish, 1954).
In this process the swelling occurs prior to osmotic (Type II) swelling which is associated with longer range electrical diffuse double layer effects. Figure 2.4.a and 2.4.b shows inner-crystalline swelling of sodium montmorillonite.
In inner-crystalline swelling, there is a balance between attractive and repulsive forces operating between adjacent interlayer surfaces (Norrish, 1954; van Olphen, 1965;
Kittrick, 1969). Electrostatic attraction between the exchange cations and the basal surfaces of the clay dominates the net potential energy of interaction (Laird, 1996 and 2006). The positive charged cations provide links or are like charge bridges between adjacent negatively charged clay layers. On the other hand, the hydration energy of the exchange cations dominates the net potential energy of repulsion. Net forces of attraction are dominant for unsaturated conditions or saturated conditions with high electrolyte concentrations, while net forces of repulsion are dominant in case of fully saturated conditions of low electrolyte concentration.
Figure 2.4.a Inner-crystalline swelling of sodium montmorillonite. Given are the layer distances and the maximum number of water molecules per sodium ion (Kraehenbuehl et al., 1987)
Figure 2.4.b The structure of water molecule
2.4.3.2 Osmotic swelling
The osmotic phase of swelling follows the hydration phase but occurs only when the exchange sites contain monovalent cations (Norrish and Quirk, 1954; Jellander et al., 1988; McBride, 1994; Prost et al., 1998). The interlayer region retains numerous layers of water molecules during the osmotic phase. The number of layers of water molecules at equilibrium is proportional to the cation concentration in the bulk water (Norrish, 1954;
Zhang et al., 1995; Onikata et al., 1999). Accordingly, when the bulk water contains a low concentration of monovalent cations and monovalent cations occupy the exchange sites, a larger fraction of the total water is bound and less mobile water is available for flow resulting in a lower value of hydraulic conductivity. This condition is commonly observed when sodium-montmorillonite are hydrated and/or permeated with DI water (Lutz and Kemper, 1958; Alther et al., 1985; Gleason et al., 1997; Petrov and Rowe, 1997; Ruhl and Daniel, 1997; Shackelford et al., 2000). When polyvalent cations occupy the exchange sites, only the hydration phase occurs. The interlayer expands until it contains four monolayers of water and then expands no further (Norrish and Quirk, 1954; Posner and Quirk, 1964; Jellander et al., 1988; McBride, 1994; Prost et al., 1998).
There are several explanations for the lack of additional interlayer swelling when polyvalent cations occupy the exchange sites, but consensus does not exist regarding which explanation is correct (McBride, 1994). Nevertheless, absence of the osmotic phase is well documented experimentally in the literature (Norrish and Quirk, 1954;
Posner and Quirk, 1964; McBride, 1994; Prost et al., 1998). Lack of an osmotic phase is evident in the free swelling of calcium-montmorillonite (i.e., bentonites where the exchange sites are occupied by Ca2+ cations), which typically is about 3 mL/2g even when DI water is the hydrating liquid. In contrast, the free swelling of sodium- montmorillonite typically exceeds 30 mL/2g in dilute monovalent solutions or DI water (Egloffstein, 1995; Lin and Benson, 2000).
In sodium-montmorillonite the swelling can result in the complete separation of the layers. The driving force for the osmotic swelling is the large difference in concentration between the ions electrostatically held close to the clay surface and the ions in the pore water of the rock (Fig. 2.5.a). Irregularities in the crystal lattice are manifested by an excess negative charge, which must be compensated by positive ions close to the surface of the clay. The concentration of positive ions close to the surface is thus extremely high, while that of negative ions is very small. The positive ion concentration decreases with
increasing distance from the surface, whereas the concentration of negative ions increases. The negatively charged clay surface and the cloud of ions form the diffuse electric double layer (Fig. 2.5.b). A high negative potential exists directly at the surface of the clay layer. The value of this potential is reduced, with increasing distance from the surface and reaches zero in the pore water. When two such negative potential fields overlap, they repel each other, and cause the observed swelling in clay. The profile of the potential curves, and therefore the repulsion at a given distance vary with the valence and the radius of the counter-ions in the double layer and with the concentration of electrolytes in the pore water. A transformation of sodium montmorillonite into its calcium form or an increase in the electrolyte concentration in the pore water results in the decrease in the double layer thickness and a reduction in the swelling stress.
Figure 2.5.a Figure 2.5.b
Figure 2.5.a Two negatively charged clay layers with ion cloud. The ion concentration C1 between the layers is much higher than the ion concentration C2 in the pore water. An equilibration of the concentration can only be reached through the penetration of water into the space between clay layers, since the interlayer cations are fixed electrostatically by the negative charge of the layers (osmotic swelling)
Figure 2.5.b Negatively charged clay surface, ions in the diffuse double layer and ions in the pore water.
The distribution of the negative potential changes with the valence and the radius of the ions in the double layer and with the electrolyte concentration in the pore water